The invention relates to a semiconductor device for generating an electron beam having a cathode comprising a semiconductor body having a p-n junction between a semiconductor surface-adjoining n-type region and a p-type region; electrons are generated in the semiconductor body by avalanche multiplication by applying a voltage in the reverse direction across the p-n junction and then these electrons emanate from the semi-conductor body.
The invention relates in addition to a method of manufacturing such a semiconductor device, as well as a camera tube and a display device having such a semiconductor device.
Semiconductor devices of the above-mentioned kind are used, for example, in cathode-ray tubes in which they replace the conventional thermal cathode in which electron emission is generated by heating. In addition to the high energy consumption due to the heating, thermal cathodes have the disadvantage that they are not immediately ready for operation because they first have to be heated before emission occurs. In addition the cathode material is lost over time due to evaporation so that these cathodes have restricted lifetimes.
In order to do away with the need for the heating source, which is cumbersome in practice, and also to remedy the other disadvantages, research has been done into producing a cold cathode.
One of the solutions was considered to be the so-called field emission cathode in which electrons were drawn from a punctiform nonheated cathode by means of a strong external electric field. However, application of this field emission cathode is very restricted due to the required very strong external electric field, the vulnerability of the cathode as a result of electric discharges in the emission space and the ultra-high vacuum (10 to 100 nano-Pascals) necessary for stable emission.
Another solution consists of so-called "negative electron affinity" cathodes in which a semiconductor body of the n-type is covered with a very thin p-type surface region and the p-n junction thus obtained is biased in the forward direction. Since the p-type surface region has a thickness which is smaller than the diffusion-recombination length of the electrons in the p-type region, electrons emitted by the p-n junction in the p-type region, provided they have sufficient energy, can emanate from the semiconductor surface at the surface of the p-type region. In order to stimulate the emanation of the electrons, the surface is usually covered with an electron work function-reducing material, for example, a cesium-containing material.
One of the problems in these "negative electron affinity" cathodes is the occurrence of recombination in the thin p-layer which restricts the injection current. Moreover, during use, the coating layer of electron work function-reducing material is slowly lost, which imposes a restriction on the life of the cathodes.
In addition to the above-mentioned cathodes, there exist cathodes which are based on the emanation of electrons from the semiconductor body when a p-n junction is operated in the reverse direction in such manner that avalanche multiplication occurs. Some electrons may obtain enough kinetic energy to exceed the electron work function potential. These electrons are then liberated at the surface and thus produce an electron beam current. Such a cathode is disclosed in British Pat. No. 1,303,659 and forms the subject matter of the present patent application. In the embodiment of the above-mentioned patent specification a cathode is described in which silicon carbide is used as a semiconductor material. In fact, only with silicon carbide in such a device is such an efficiency obtained, that is to say such a ratio between the generated electron current and the required avalanche current through the p-n junction, that the device is useful for practical application.
When the above-mentioned kinds of cathodes are used, for example, in camera tubes or small display tubes, the released electrons are accelerated by means of control grids and the electron beam thus obtained is often concentrated at a point by means of electron optics. This point, a so-called "cross-over", serves as a real source for the actual electron beam which is then deflected, for example by electromagnetic means such as deflection coils, to scan in a camera tube a photoconductive layer which contains image information.
At the area of the above-mentioned "cross-over" mutual interactions take place between the released electrons. The distribution of the electron energy is altered so that the associated electron temperature is increased and the energy distribution of the electrons becomes larger. This has a detrimental influence on the so-called acceptance curve of the beam in the sense that after-effects occur in the camera tube.
The device described in British Pat. No. 1,303,659 shows a p-n junction which intersects the surface of the device. When the semiconductor device is incorporated in a cathode-ray tube or another discharge device, the cathode will generally form part of a larger assembly in which, as a result of other electrodes, for example an anode or control grids, the generated electrons are drawn away in a direction perpendicular to the major surface of the semiconductor device. Considered in a broader sense the electrons are thus subjected to an electric field having a component perpendicular to the major surface. Major surface is to be understood to mean herein the semiconductor surface including grooves or recesses, if any.
The electric field of the p-n junction emitting electrons as a result of avalanche breakdown is directed perpendicular to the p-n junction. As a result of this, the emanated electrons may have a velocity component in a direction other than the desired direction. This may be disadvantageous, in particular when a narrown electron beam is required.
Furthermore, experiments have demonstrated that in devices of the type described in the British patent specification in which the depletion zone associated with the p-n junction adjoins the surface the electrons generated by means of such a cold cathode have an energy distribution which is not optimum, notably for use in a camera tube. The released electrons as a matter of fact have no sharply defined electron temperature but the energy distribution of the emitted electrons shows a second wide distribution of energy values in addition to a sharp peak which depends on the current through the p-n junction and the voltage on the accelerating electrode. Such an energy distribution detrimentally influences the above-mentioned acceptance curve of the beam.
The form of the energy distribution can presumably be explained as being made up of two distributions. The wide distribution is due to the emanation of electrons which obtain sufficient energy in the depletion region to exceed the electron work function potential and hence emanate from different points on the surface with different potentials. The narrow peak on the other hand occurs mainly by the emanation of electrons which have traversed the whole depletion zone and emanate from the conductive part of the n-type zone which has a substantially constant potential at the surface.